THE CIRCULATION OF THE BLOOD. 
109 
one another, and at that point a small aperture brings the 
two into communication. The venous and arterial cur¬ 
rents are, therefore, mixed, 
but not within the heart, as 
in the other Reptiles, nor so 
extensively. In the structure 
of the heart, as well as in that 
of the gizzard, Crocodiles ap¬ 
proach the Birds. 
The Highest Form of the 
Circulating System is pos¬ 
sessed by the warm-blooded 
Vertebrates, Birds and Mam- fig. 73—Heart of the uugoug, a four- 
1 vr i chambered heart, the parts being more 
mals. JNOt a drop OI blood separated than in higher animals: E, 
.v.right ventricle; L, left ventricle; D, 
can make the circuit ot the rl j hl >nricle . F ,p U , monal . y aru;ry : 
body without passing through K > left aiiriele; A » aorta - 
the lungs, the circulation to and from those organs being 
as perfect as the distribution of arterial blood. The heart 
/ 3 * 9 consists of four cavities — a 
\ » i 
right auricle and ventricle, and 
a left auricle and ventricle. In 
other words, it is a hollow mus¬ 
cle divided internally by a ver¬ 
tical partition into two distinct 
chambers, each of which is 
again divided by a valve into 
an auricle and a ventricle. The 
work of the right auricle and 
Fig. 74. -Theoretical Section of the ven f r j 0 l e [a f 0 receive the blood 
Human Heart: a, right ventricle; v enhriOie ictene Uie UlOOU 
b, inferior vena cava; c, tricuspid from tile veins, and Send it to 
valve; d, right auricle; e, pulmona¬ 
ry veins; /, superior vena cava; g, the lllilgS ; while the Other two 
pulmonary arteries; h, aorta; k, left . . 
auricle ; l, mitral valve; m, left ven- receive the blood irom tile 
tricie; «, septum. lungs, and propel it over the 
body. The left ventricle has more to do than any other 
cavity. The two auricles contract at the same instant; 
d 
